Mr Nel is now working with the Afriforum group, which mainly lobbies for the rights of Afrikaners in South Africa.

Afriforum said if the police failed to act in the case then it would take up a private prosecution.

It also said that it would fight any move to grant Mrs Mugabe diplomatic immunity.

South Africa’s diplomatic dilemma

Farouk Chothia, BBC Africa

South Africa’s government risks a public backlash if it lets Mrs Mugabe go scot-free.

This happened in 2015, when it failed to execute an international arrest warrant for Sudan’s President Omar al-Bashir who was wanted by the International Criminal Court.

The government argued that he qualified for diplomatic immunity, but the country’s judges disagreed. The government was then strongly criticised for undermining the rule of law.

It seems that ministers want to avoid a similar backlash and are therefore insisting that Mrs Mugabe must appear in court.

But by taking such an approach it risks a diplomatic row with Zimbabwe’s government – a staunch ally whom it has resolutely defended over the years, despite international criticism of President Mugabe’s human rights record.

So the two governments, including Mr Mugabe and his South African counterpart, Jacob Zuma, are bound to be in talks to resolve the crisis over the first lady.

One option being mentioned in the South African media is that Mrs Mugabe should plead guilty during a short court appearance, and pay a fine.

But it is unclear whether Mr and Mrs Mugabe known for their uncompromising nature will agree to this, especially after Zimbabwe’s ruling Zanu-PF party said in a tweet on Tuesday that the first lady had been “attacked”, contradicting her accuser’s version of events.